See mod described below.

I changed colors at 2.6mm.

I used PLA, a 0.25mm nozzle and layer height of 0.15mm, 3 contours, 15% infill, no support.

To open and lock I advise you to tilt two skull horizontally, smoother not crossing layers, less wear.

I added an external reinforcement in case you use a skull made of TPU as a lock (less sensible to wear).